1. Field of the Invention
The invention relates to a device for preventing the memory effect in cryogenic pumps according to the pre-characterising clause of claim 1. The invention also relates to a cryogenic pump according to claim 11.
2. Prior Art
Cryogenic pumps operated with a two-stage cold head are distinguished by a high pumping capacity and are used to generate an ultra-high vacuum (p<10−7 mbar). Such pumps have been commercially available for over 30 years.
The pumping surface areas of the first stage are usually constructed as a cup-shaped shield and as a double-conical baffle in the region of the cup opening. The pumping surface areas of the first stage should be kept at about 80 Kelvin and serve to freeze vapour and gases with similar resublimation points.
Gases with lower resublimation points freeze on the pumping surface areas of the second stage whose temperature is less than 20 K.
At the transition from the first stage to the second stage the base of the cup-shaped shield is centrally penetrated by the cold head. Temperature zones of about 30 K consequently result in the immediate surroundings of the connecting point between cold head and base of the shield at the base.
What is referred as a memory effect is known in the case of cryogenic pumps with a two-stage cold head. There are gases which liquefy at the above-described temperature zones of about 30 K. The liquefied gases have a vapour pressure which counteracts the ultra-high vacuum which has been generated. A vacuum is established as a result of the vapour pressure and it is no longer possible to fall below this during continuous operation of the cryogenic pump. The higher the concentration of such gases which can liquefy at about 30 K is in the atmosphere to be removed by suction, the more serious the memory effect is on the vacuum to be achieved.
Two proposals are known commercially (which are designed) to prevent this memory effect. Firstly the temperature zones at the base of the shield are heated in an obvious manner by heating elements to the temperature of the first stage of 80 K.
Secondly, a thermal bridge is known in the case of cryogenic pumps which conduct heat from the housing of the cryogenic pump to the temperature zones of the base of the shield, so the memory effect is likewise prevented.
The drawback of these proposals is that heat is in each case supplied from outside to the cold head during operation of the cryogenic pump and the cooling capacity of the second stage, and consequently the efficiency of the cryogenic pump, is reduced.
The present invention provides a cryogenic pump that does not exhibit the drawback described above. In other words, the present invention provides a cryogenic pump that does not have the memory effect.